The device of the present invention relates to thermal imaging systems, and, more particularly, to detector dewar assemblies used in such systems.
In most thermal imaging systems using semiconductor detection devices which must be cryogenically cooled, it is necessary to provide a housing (or "dewar") for the detector device. The dewar must be evacuated and provide electrical connections between the detector device and the housing between the detector array and the housing exterior so that the device may be interfaced to other signal processing or imaging subassemblies.
Typically, these dewars include several concentric cylindrical tubes or housings. The inner tube is generally comprised of a long cylindrical glass bore which opens to the base of the dewar. This bore, often referred to as the "coldwell", is multifunctional. First, the detector device or array is mounted at the top of the coldwell (the "endwell"). A "cold finger" or cryogenic cooling device is generally inserted into the bore of the coldwell to effect cooling of the detector array. Often the exterior surfaces of the coldwell are metalized and then etched or delineated in order to define conductive leads which run the length of the bore.
Past dewars have used a gold plated glass bore to provide a thermally reflective shield and reduce heat load. In addition, glass has been used because it has the advantage that it is a good electrical and thermal insulator, it is not ductile and once fixed in the system, alignment will remain true unless the bore breaks. However, the glass coldwell has the disadvantage that it is extremely fragile and requires more complex and expensive assembly techniques.
In prior art dewars, the base portion of the coldwell supports a flange to which a cap is attached. This cap is typically made of Kovar or some other metal which has appropriate structural and thermal properties and can maintain a hard vacuum. The top part of the cap directly above the detector array holds a small, transmissive window.
In one type of dewar, this flange comprises a ceramic material. The horizontal ceramic substrate receives wire connectors from the vertical glass coldwell (jumper wires). The ceramic conductor pattern (usually radial) connects through the vacuum wall to conventional pin connectors outside the vacuum portion of the dewar. Thus, the detector leads can be connected to external imaging or signal processing subassemblies.
In a second dewar design, a metal flange, bonded appropriately to the glass coldwell, serves primarily as a supporting member for the vacuum chamber cap and window. The base portion of the glass coldwell is thickened so that it may include buried conductive wires which surface above and below the flange providing an electrical exit from the vacuum chamber. Gold jumper wires are used to connect the axial leads on the coldwell to the detector array and to surfaced conductors on the coldwell above the flange. The conductors surfacing below the flange (outside the vacuum) may be connected to a conventional tape cable or other wiring device.
In either design a second larger flange is connected to the coldwell at its base. This flange supports an outer protective housing which encloses the vacuum chamber, as well as supporting the mounting flange for the dewar. The housing has openings for the end of the vacuum cap with transmissive window, and for electrical connectors. In either design, a tape cable or other wiring device connects from the vacuum exit conductors to standard electrical interconnects.
These prior designs have several disadvantages. First, to repair the detector array or other connections within the vacuum chamber the entire device must be disassembled; the outer housing removed, wire cable detached, and the vacuum cap removed. Second, since the coldwell is made of glass, the device is particularly susceptible to breakage during assembly, disassembly, and normal use. Third, the gold jumper wire connectors used at the array and flange or the buried lead connections are susceptible to breakage. Fourth, the volume of the vacuum chamber is usually a small percentage of that of the finished assembly, such that when impurities from the materials outgas into the vacuum chamber, the vacuum degrades at a high rate. Finally, because the design uses many parts and requires use of more complex assembly techniques, the cost per unit is usually high.
Accordingly, it is a primary object of the present invention to provide an improved detector dewar assembly.